In a wireless communication system, a base station could provide one or more coverage areas, such as cells or sectors, in which the base station could serve user equipment devices (UEs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality (whether or not operated by a human user). In general, each coverage area could operate on one or more carriers each defining a respective bandwidth of coverage, and each coverage area could define an air interface providing a downlink for carrying communications from the base station to UEs and an uplink for carrying communications from UEs to the base station. The downlink and uplink could operate on separate carriers or could be time division multiplexed over the same carrier(s). Further, the air interface could define various channels for carrying communications between the base station and UEs. For instance, the air interface could define one or more downlink traffic channels and downlink control channels, and one or more uplink traffic channels and uplink control channels.
In accordance with the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS), for instance, each coverage area of a base station could operate on one or more carriers spanning 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz. Each of these carriers is subdivided in both time and frequency to define an array of resource elements for carrying communications between the base station and UEs, with each resource element occupying a 15-kHz-wide subcarrier and 66.7 microseconds (μS). In particular, each carrier is divided in the frequency domain into groups of twelve 15-kHz wide subcarriers. And in the time domain, each 15-kHz-wide subcarrier is divided into a continuum of 10-millisecond (ms) frames, with each frame being further divided into ten 1-ms subframes that are in turn each divided into two 0.5-ms slots. Slots are each further sub-divided into a number (typically seven) of symbol times, with a single resource element corresponding to a symbol time of a particular subcarrier. In such an arrangement, each 1-ms subframe of a particular subcarrier includes fourteen 66.7-μs long resource elements plus a 4.69-μs guard band (cyclic prefix) per resource element to help avoid interference between resource elements.
Within a subframe, different resource elements can serve different functions. For instance, on the downlink, certain resource elements across the bandwidth of a carrier can be reserved for control signaling. In practice, resource elements of the first one, two, or three symbol times of a subframe are reserved to define various control channels, such as a Physical Downlink Control Channel (PDCCH) for carrying control signals such as page messages and resource allocations from a base station to UEs, a Physical Hybrid Automatic Repeat Request Indicator Channel (PHICH) for carrying messages indicating whether the base station received uplink transmissions from UEs, or a Physical Control Format Indicator Channel (PCFICH) for carrying signaling overhead information such as an indication of how many 66.7-μs time segments are being used for control signaling. Other resource elements can then be reserved for non-control signaling. For instance, the remaining resource elements outside of the control region (e.g., outside of the first one, two, or three symbol times) can be reserved to define a Physical Downlink Shared Channel (PDSCH) as a primary channel for carrying data from the base station to UEs on an as-needed basis. There can be some exceptions to this arrangement, as certain control signals, such as a reference signal useable by UEs to detect and evaluate coverage, may be occasionally allocated to resource elements outside of a subframe's control region.
Each resource element can represent a number of bits, with the number of bits depending on how the data is modulated. The data could be modulated according to various modulation and coding schemes (MCSs) having various orders of modulation. For instance, with Quadrature Phase Shift Keying (QPSK) modulation, each resource element represents 2 bits; with the higher-order 16 Quadrature Amplitude Modulation (16QAM), each resource element represents 4 bits; and with the even higher-order 64QAM, each resource element represents 6 bits. Other examples of MCSs can be used to modulate the data as well.